In a fossil fired steam generator, the energy of a fossil fuel is used to produce superheated steam which in a power plant, for example, can then be supplied to a steam turbine for power generation.
Particularly at the steam temperatures and pressures prevalent in a power plant environment, steam generators are normally implemented as water tube boilers, i.e. the water supplied flows in a number of tubes which absorb energy in the form of radiant heat of the burner flames and/or by convection from the flue gas produced during combustion.
In the region of the burners, the steam generator pipes here usually constitute the combustion chamber wall by being welded together in a gas-tight manner. In other areas downstream of the combustion chamber on the flue gas side, steam generator pipes disposed in the waste gas flue can also be provided.
Fossil fired steam generators can be categorized on the basis of a large number of criteria: steam generators may in general be designed as natural circulation, forced circulation or continuous steam generators. In a continuous steam generator, the heating of a number of steam generator pipes results in complete evaporation of the flow medium in the steam generator pipes in one pass. Once evaporated, the flow medium—usually water—is fed to superheater tubes downstream of the steam generator pipes where it is superheated.
Strictly speaking, this description is valid only at partial loads with subcritical pressure of water (PKri≈221 bar) in the evaporator—at which there is no temperature at which water and steam can be present simultaneously and therefore also no phase separation is possible. However, for the sake of clarity, this representation will be used consistently in the following description. The position of the evaporation end point, i.e. the location at which the water content of the flow is completely evaporated, is variable and dependent on the operating mode. During full load operation of a continuous steam generator of this kind, the evaporation end point is, for example, in an end region of the steam generator pipes, so that the superheating of the evaporated flow medium begins even in the steam generator pipes.
In contrast to a natural or forced circulation steam generator, a continuous steam generator is not subject to pressure limiting, so that it can be designed for main steam pressures well above the critical pressure of water.
During light load operation or at startup, a continuous steam generator of this kind is usually operated with a minimum flow of flow medium in the steam generator pipes in order to ensure reliable cooling of the steam generator pipes. For this purpose, particularly at low loads of for example less than 40% of the design load, the pure mass flow through the evaporator is usually no longer sufficient to cool the steam generator pipes, so that an additional throughput of flow medium is superimposed in a circulatory manner on the flow medium passing through the evaporator. The operatively provided minimum flow of flow medium in the steam generator pipes is therefore not completely evaporated in the steam generator pipes during startup or light load operation, so that unevaporated flow medium, in particular a water-steam mixture, is still present at the end of the evaporator pipe.
However, as the superheater tubes mounted downstream of the steam generator pipes of the continuous steam generator and usually only receiving flow medium after it has flowed through the combustion chamber walls are not designed for a flow of unevaporated flow medium, continuous steam generators are generally designed such that water is reliably prevented from entering the superheater tubes even during startup or light load operation. To achieve this, the steam generator pipes are normally connected to the superheater tubes mounted downstream thereof via a moisture separation system. The moisture separator is used to separate the water-steam mixture exiting the steam generator pipes during startup or light load operation into water and steam. The steam is fed to the superheater tubes mounted downstream of the moisture separator, whereas the separated water is returned to the steam generator pipes e.g. via a circulating pump or can be drained off via a flash tank.
Based on the flow direction of the gas stream, steam generators can also be subdivided, for example, into vertical and horizontal types. In the case of fossil fired steam generators of vertical design, a distinction is usually drawn between single-pass and two-pass boilers.
In the case of a single-pass or tower boiler, the flue gas produced by combustion in the combustion chamber always flows vertically upward. All the heating surfaces disposed in the flue gas flue are above the combustion chamber on the flue gas side. Tower boilers offer a comparatively simple design and simple control of the stresses produced by the thermal expansion of the tubes. In addition, all the heating surfaces of the steam generator pipes disposed in the flue gas flue are horizontal and can therefore be completely dewatered, which may be desirable in frost-prone environments.
In the case of the two-pass boiler, a horizontal gas flue leading into a vertical gas flue is mounted in an upper region downstream of the combustion chamber on the flue gas side. In said second vertical gas flue, the gas usually flows vertically from top to bottom. Therefore, in the two-pass boiler, multiple flow baffling of the flue gas takes place. Advantages of this design are, for example, the lower installed height and the resulting reduced manufacturing costs.
In a steam generator embodied as a two-pass boiler the walls are generally arranged suspended in a boiler framework, so that upon being heated during operation it can expand freely downwards. The two-pass stem generator here generally comprises four walls per flue, where it should be ensured that the walls of the individual flues expand evenly, as impermissible tensions can otherwise occur in the connections of the walls.
Frequently, two-pass boilers of this kind further comprise a so-called combustion chamber nose. This nose is a projection, which is formed from the combustion chamber wall inclined inwards at the transition to the horizontal gas flue and the bottom of the horizontal gas flue. A combustion chamber nose of this kind improves the flow of flue gas at the transition to the horizontal gas flue.
It is however disadvantageous that the pipework of the combustion chamber rear wall, that is the wall facing the horizontal gas flue and the second vertical gas flue is interrupted by the combustion chamber nose. The weight of the rear wall must thus generally be passed into the boiler framework between the upper and lower end of the nose by means of a special construction in such a way that upon heating or loading—for example as a result of internal pressure, ash build-up or its own weight—the rear wall moves to the same degree as the other walls. To date there have been various approaches to the solution of this problem:
For example the upper and the lower end of the nose can be effected by means of flue rods and springs or so-called constant hangers, which despite changes to the spring deflection always transfer approximately the same force. A construction of this kind thus adapts to the differential expansion of the walls. Different loads for example as a result of changing internal pressure or ash build-up do however give rise to high levels of tension at the connections to the side walls. In addition, these constant hangers are costly.
A further possibility lies in the in the simple continuation of the pipes of the lower combustion chamber in a vertical direction as far as the suspension point in the boiler framework. The connection from the lower end of the nose to the boiler framework thus has approximately the same temperatures as the side walls and the front wall. The pipework of the nose must though then be embodied in separate form, which means an additional outlay in terms of connecting pipes.
A further possibility lies in dividing the pipes of the combustion chamber rear wall at the lower end of the nose on the flow medium side, so that a part of pipes are routed into the pipework of the nose, another part parallel to this vertically to the boiler framework. Therefore, however, only part of the pipes and of the flow medium is available to the nose, which can under certain circumstances lead to inadequate cooling of the nose, as the latter has a comparatively high heat input through its exposed position in the combustion chamber. In contrast to this, the heat input for the support pipes removed and routed vertically upwards is correspondingly lower, which can give rise to problems in relation to the distribution of the mass flow. All wall pipes above the nose and the support pipes should if possible have the same steam temperatures at the outlet. Furthermore a laborious transition into the nose pipework for example by changing the division of the pipes or other pipe geometry is required.